1. Field of Invention
The invention relates to a method for generating header and transition flags using the DPD (Differential Phase Detection) technology and an optical device using the same.
2. Related Art
In the DVD-RAM (Digital Versatile Disk RAM) structure, there are wobbled spiral groove and land tracks, and the phase change recording method is used at the centers of groove and land to reach the high density recording capacity. FIG. 1 shows such a track structure of wobbled grooves 13 and lands 14.
As shown in the drawing, each groove/land track is divided into several sectors. The user""s data are continuously recorded on the groove/land track in units of ECC (Error Code Correction) blocks. Each ECC block consists of 16 sectors. Each sector has previously etched with a header field, containing a first/second header field 11, a third/fourth header field 12, and having Half-track pitch offsets in the radial direction. The header field of each sector has the same embossed pit/land structure as the DVD-ROM and contains CAPA""s (Complementary Allocated Pit Address). Furthermore, as shown in FIG. 1, at the intersection of each loop of groove and land (such as the position 15 in the drawing) the last sector of the groove (land) connects to the first sector of the land (groove), and at the same time the polarities of the first/second header field 11 and the third/fourth header field 12 are changed.
To read the data in the DVD-RAM, the reading system has to use the header flag and the groove/land transition flag to control the reading actions. Nowadays, the so-called high frequency push-pull tracking method is used to generate the header flag signals and the groove/land transition flag signals according to the embossed header field. FIG. 2 shows the push-pull CAPA signal generated by following the groove and land tracks, where the polarities of FIGS. 2(A) and 2(B) are opposite to each other. As shown in the drawing, the level of the push-pull CAPA signal is very low at locations other than the header field. At the header field, the level of the push-pull CAPA signal increases and changes its sign according to the polarity of the header field.
Therefore, the system can only use a low-pass filter to filter the push-pull CAPA signal to generate the signal 21 as shown in FIGS. 3(A) and 3(D). After comparing with high and low threshold levels 22, 23, the header flag signals 26, 31 and the groove/land transition flag signals 24, 25 are readily obtained. If the order of the groove/land transition flag signals generated by the system is 24xe2x86x9225, it means that the groove data are being read and the system has to track the groove. On the other hand, if the order of the groove/land transition flag signals generated by the system is 25xe2x86x9224, it means that the land data are being read and the system has to track the land.
However, the high-frequency push-pull tracking signal is very sensitive to the lens shift, thus greatly influencing the header flag signal and groove/land transition flag signal generating method as described in FIG. 3. FIG. 4 shows the deformed wave of the push-pull CAPA signal filtered by the low-pass filter due to lens shifts. As shown in the drawing, it is impossible to generate groove/land transition flag signals 24, 25 according to the high and low threshold levels because of the push-pull CAPA signal deformation. Furthermore, due to the lack of the transition flag signal 24 or 25, it is also impossible to generate complete header flag signals 26, 31. This will result in the control system""s being unable to function correctly.
In view of the foregoing, an object of the invention is to provide a method for correctly generating groove/land transition flag signals without being influenced by lens shifts.
Another object of the invention is to provide a method for generating groove/land transition flag signals using the DPD technology.
To achieve the above objects, the invention uses the DPD technology to generate header flags. The disclosed method includes the steps of: setting first and second threshold levels; generating a phase difference signal, i.e. starting a phase detection function to generate a phase difference signal; generating transition flag signals with a first transition flag signal and a second transition flag signal, where the first transition flag signal is HIGH if the phase difference signal is greater than the first threshold level, and the second transition flag signal is HIGH if the phase difference signal is smaller than the second threshold level; and generating a header flag signal according to the transition flag signal.
Using the DPD technology has the following advantages:
1. The DPD technology only produces a correct phase difference signal for an embossed pit/land structure and has a relatively small phase difference signal for a phase change structure. Furthermore, the DVD-RAM format contains the phase change areas recorded with user""s data and embossed pit/land areas recorded with CAPA signals. Therefore, the phase difference signal can clearly distinguish the embossed pit/land areas and the phase change area.
2. Since the DPD technology is less sensitive to lens shifts, no phase difference signal errors will be generated by lens shifts.